The present invention is concerned with piston rings for internal combustion engines, and particularly with oil control rings. More specifically, the present invention is concerned with a rolled steel oil control piston ring for internal combustion engines. However, the invention is not limited thereto, and concerns piston rings in general.
Oil control rings for reciprocating piston internal combustion engines comprise split, generally circular rings which may be U-shaped in cross section. The rings are adapted to be received in a circumferential groove formed in the piston, with the free ends of the legs of the U (the sidewalls of the ring) projecting inwardly of the ring towards the bottom of the groove, and the connecting central portion of the U (the peripheral wall of the ring) facing outwardly of the groove, towards the cylinder wall. The outwardly facing peripheral wall has scraper rims formed thereon and the ring is thrust outwardly by an expander spring so that the rims engage the cylinder walls. The expander spring usually is disposed in the piston groove between the piston ring and the bottom of the groove, and engages the inside of the piston ring to force it outwardly.
Such oil control rings having a pair of flat-faced scraper rims to engage the cylinder walls, also have vents between the rims of the ring to admit the passage of debris therethrough, as understood by those skilled in the art.
For example, U.S. Pat. No. 3,378,268 assigned to the assignee of the present application, shows in FIG. 1 thereof an oil control ring assembly comprising a one-piece cast iron ring and an expander spring engaged therewith. The expander spring is between the bottom of the piston groove and the oil control ring and serves to expand the ring outwardly into contact with the cylinder wall of the engine. The cast-iron ring has rims formed in the peripheral wall thereof. The rims have flat scraping faces which contact the cylinder wall. The desired configuration of the ring is obtained by providing a casting of suitable shape which is thereafter machined to finished dimensions. Generally, castings are employed in the piston ring field, although it is conceivable that a ring of the desired configuration may be manufactured by a forging operation.
However, the use of cast iron oil control rings has some disadvantageous limitations. The degree of hardness which can be attained in a cast iron ring is limited by the ability to economically machine high hardness parts. If heat treatment to harden previously machined parts is used, part distortion of the small section rings is a problem. Further, cast iron rings are somewhat prone to damage upon installation within the groove of a piston. In addition, the expander spring normally employed in conjunction with the oil control ring is usually made of steel and, being harder than the cast iron ring, tends to wear those portions of the ring which it contacts to expand the ring.
The foregoing limitations can be avoided by forming the oil control ring from a hard metal such as steel. The manufacture of oil control rings by rolling from metal strip stock is also suggested in U.S. Pat. No. 3,378,268. For example, in FIGS. 5 and 7, a rolled oil ring formed from strip metal is shown. It will be appreciated that rolling the ring to proper profile from strip metal and thereafter coiling and cutting rings from the coiled, profiled strip is an efficient and much less expensive method of manufacture than the use of machined castings or of forgings.
The aforesaid patent also illustrates a prior art attempt to correct for large dimensional variations of rolled as opposed to machined cast rings. This attempt is to make the rolled ring from stock which is thin and flexible enough so that dimensional variations could be overcome by deformation of the rings against the cylinder wall surfaces by the expansion force of the expansion spring. (See FIG. 1 of U.S. Pat. No. 3,378,268.) Rings rolled from such relatively thin stock however, are inferior to rings of more rigid stock in terms of ring positional stability and oil scraping characteristics.
It is accordingly an object of the present invention to overcome these and other shortcomings of the prior art.
It is an object of the present invention to provide a rolled metal piston ring, and in particular a rolled metal oil control piston ring which provides rings of closely controlled dimensional variations. It is another object of the invention to provide such a rolled ring which is of rigid construction, being made from relatively heavy stock to impart desired rigidity and improved scraping action to the ring, as compared to the structure of U.S. Pat. No. 3,378,268.
In accordance with the present invention there is provided a piston ring comprising a rolled metal, preferably steel, ring of generally U-shaped cross section having at least one scraping rim formed in the peripheral wall thereof by a pleat folded from the wall and at least partially closed on itself. In a preferred embodiment, the rims are flatfaced. There are two such rims provided, and they extend circumferentially around the peripheral wall of the ring, which also contains vent holes in the ring, between the scraper rims. The U-shape cross section is provided by a centrally disposed peripheral wall from which opposed sidewalls project inwardly of the ring.
After the initial forming of the U-shaped cross section profile, the strip is passed through an extruder-like work space between opposed roll forms in which it is subjected to sufficient pressure to impose an extrusion effect upon the metal by which the cross sectional area of the strip is reduced slightly to form the final cross sectional dimensions thereof.
In accordance with one aspect of the invention, the pleats are formed so that the finished rims have flat scraping faces.
The completed profiled strip is punched to form vent holes therein, and coiled into a helical coil to give a permanent coiled set to the strip. The coils may be severed to any desired coil length for storage or further processing. The scraper rims or the outer face of the ring including the scraper rims, may be provided with a wear resistant coating such as a chromium plated coating or a molybdenum coating. The coiled strip is then cut longitudinally so that a plurality of generally circular, gapped annular rings is formed therefrom.